1. Technical Field of the Invention The present invention relates generally to an ac electric motor for use in passenger automobiles or autotrucks, and more particularly to such a motor designed to ensure high efficiency in operation and to be allowed to be reduced in size and manufactured at low costs.
2. Background Art
Japanese Patent First Publication No. 6-261513 discloses a brushless motor equipped with multiple phase windings each of which wound around a stator magnetic pole in the form of a concentrated winding. This structure encounters the drawback in that each of the windings needs to be placed deep in one of slots between the stator magnetic poles, thus resulting in limited numbers of turns of the windings, and has difficulties in permitting the motor to be reduced in size and produced at low costs and in improving torque-producing efficiency.
In order to alleviate such problems, the inventors of this application have proposed an improved structure of an ac motor in Japanese Patent First Publication No. 2005-160285 that is an equivalent to US2005/0099082 A1 filed Nov. 8, 2004. This will be described below.
The ac motor includes a rotor, a stator, and a plurality of looped windings. The rotor has north and south poles arranged alternately along a circumference thereof. The stator has stator magnetic poles arranged along a circumference thereof. The stator magnetic poles are broken down into N phase pole groups which are located out of alignment in circumferential and axial directions of the stator. The looped windings extend in the circumferential direction of the stator in magnetic relation to the N phase pole groups and are arranged adjacent each other in the axial direction of the stator. Considering an example in which windings wound around the stator magnetic poles of the same group, currents which flow through portions of the windings lying between adjacent two of the stator magnetic poles work to produce magnetomotive forces each of which cancels the other. This is equivalent to when no current flow through the portions of the windings. It is, thus, possible to replace the windings wound around the stator magnetic poles with the looped windings. This means that the portions of the windings lying adjacent two of the stator magnetic poles are omitted, thus permitting the motor to be designed to produce increased torque and the stator magnetic poles to be increased. The structure of the looped windings is simple, thus resulting in improved productivity thereof. The looped windings extend symmetrically with respect to the circumference of the stator, thus resulting in a decrease in deformation of the stator or strain of parts of the motor arising from magnetic attraction, as produced between the rotor and the stator, which leads to reduction in mechanical vibration and noise in the motor.
The inventors of this application has studied the number of the stator magnetic poles and the layout thereof in the above type of ac motor in order to increase output torque, decrease torque ripples, and/or simplify the structure of the motor further.